1. Field of the Invention
The present invention relates generally to a communications system for automotive vehicles, and more particularly to a system including a mobile communications unit for communicating with a fixed communications unit which transmits necessary information to the mobile unit whenever an automotive vehicle provided with the mobile unit passes through the area in which it is possible to communicate between the mobile unit and the fixed unit (hereinafter referred to as "communications area," simply).
2. Description of the Prior Art
There is a need for an automotive vehicle to communicate with fixed communications units equipped on appropriate roadside buildings, in order to receive necessary useful information. Therefore, a communications system is necessary between the mobile unit and the fixed unit. In a communications system of this type, a fixed unit fitted on a roadside building transmits an area signal at all times to inform the mobile units on automotive vehicles of the presence of a communications area. When passing through the communications area, a mobile unit on an automotive vehicle receives the area signal and immediately transmits back a request signal to the fixed unit. When receiving the request signal from a mobile unit, the fixed unit begins to transmit back an information signal (possibly corresponding to the request signal) to the mobile unit.
In this communications system, usually, a threshold level for electric field strength transmitted from the fixed unit is predetermined at a constant level in the mobile unit, for the purpose of obtaining a certain fixed minimum S/N ratio.
Although almost constant within a small area, the electric field strength transmitted from the fixed unit is susceptible to change because of the influence of ambient conditions, for example, such as multi-path transmission due to reflections from other vehicles or buildings. That is, drops in the electric field strength readily occur. Thereby, communications is often interrupted between the mobile unit and the fixed unit, even after the fixed unit has begun to transmit information signals.